Yakovlev Yak-42 Yak-42LL / Yak-42E-LL

During the 1980s, with fuel at the centre of every airline calculation, the Soviet industry chased the propfan — an unducted contra-rotating engine promising far lower consumption than a comparable turbofan — as hard as its American counterpart. The Progress D-236 was the Soviet answer, and it needed an aircraft to take it aloft.

A Yak-42 was the natural choice: its side engines hang off the rear fuselage and can be swapped without touching the wing. The D-236, rated at 8,090 kW, replaced the starboard engine on a deliberately tall pylon, since its 4.2 m propellers would not otherwise clear the fuselage. The rest of the powerplant stayed standard, so the aircraft flew with two turbofans and one propfan at once — the ideal arrangement for comparing them under identical conditions.

The aircraft first flew on 15 March 1991 and was shown publicly at the Paris Air Show, at the height of the Soviet industry's opening to the West. The test programme measured fuel burn, noise and vibration, the three fronts that would decide whether a propfan could ever reach an airliner.

Cheap fuel and collapsing post-1991 budgets killed the propfan's commercial prospects on both sides of the Atlantic. The testbed remains one of the few aircraft that actually flew the technology, and one of the most striking conversions in the whole Yak-42 family.